The present invention relates to the repair of crust holes in electrolytic cells for metal production. Such cells are commonly used for producing aluminum by electrolytic reduction of alumina dissolved in a molten electrolyte.
Production of aluminum by electrolysis of alumina is a well-known process. Commercial aluminum production is carried out in a reduction cell by the Hall-Heroult process in which alumina is dissolved in molten cryolite at about 960-980xc2x0 C. An electric current passing through the molten electrolyte reduces alumina to aluminum which is collected in a pool beneath the molten electrolyte bath.
Electric current enters the cell through an anode in contact with the molten electrolyte, passes downward through the electrolyte, through the pool of molten aluminum, and into a cathode which is formed integrally with the cell bottom. The current leaves the cell through a metal collector bar below the cell bottom and is conducted to an anode in the next of a series of cells making up a pot line.
The cell operating temperature is maintained by resistance heating of the molten electrolyte, electrochemical reactions occurring in the cell, and insulating the cell structure. A frozen crust above the molten electrolyte helps to reduce heat loss because of its insulating effect.
However, the solid crust must be broken periodically to remove molten metal from the metal pool by a vacuum tap. This is achieved by periodically breaking through the solid crust with a crust breaker apparatus. The broken crust collapses down into the molten electrolyte and melts, leaving a hole in the crust above the electrolyte. The crust is also broken periodically when carbon anodes are replaced.
Breaking the solid crust produces holes that increase heat loss from the molten electrolyte. For example, a hole in the crust having an area of only 1 square foot increases heat loss sufficiently that the cell voltage must be increased by about 100 millivolts to maintain cell temperature.
Accordingly, there is a need for a means of repairing crust holes in order to reduce heat loss from the molten electrolyte.
A principal objective of the present invention is to provide an efficient and economical process for repairing crust holes in an electrolytic cell.
A related objective of the invention is to provide a crust hole repair process that avoids contaminating the electrolytic cell with substances that might interfere with cell operation or increase metal impurity levels.
A further objective of the invention is to provide a crust hole repair process that does not pose health or safety risks to pot line workers.
An advantage of the invention is that it offers a means for recovering alumina values in ESP dust collected at alumima refineries and aluminum smelters.
Additional objectives and advantages of our invention will become apparent to persons skilled in the art from the following detailed description.
In accordance with the present invention there is provided an electrolytic cell wherein a metal is produced by electrolysis. A preferred cell for production of aluminum by electrolysis of alumina comprises a pot defining a chamber containing a molten electrolyte, a cathode, at least one anode contacting the electrolyte, and a solid crust above the electrolyte. The crust comprises solidified electrolyte and alumina, and may build up to a thickness of several inches.
The molten electrolyte comprises sodium fluoride and aluminum fluoride in a weight ratio of about 0.7-1.2, together with lesser amounts of magnesium fluoride and calcium fluoride. The molten electrolyte has a temperature of at least about 900xc2x0 C., more preferably about 900-1050xc2x0 C. The electrolyte is preferably maintained at a temperature of about 960-980xc2x0 C. As reduction takes place, a pad of molten aluminum settles on the cell bottom, above the cathode.
In order to tap molten aluminum from the cell periodically the crust is broken, leaving a hole through which heat is lost from the electrolyte. Cell voltage must then be increased to compensate for the heat loss, resulting in increased electric power consumption.
Heat loss through the crust is reduced by repairing the crust hole. The hole is covered with a receptacle containing a flowable mass of solid particles. The receptacle is preferably closed.
The receptacle comprises a polymeric material. As used herein, the term xe2x80x9cpolymeric materialxe2x80x9d includes cellulosic materials, polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate, and polyamides.
A preferred receptacle comprises a cellulosic material, more preferably one or more of paper, polymer-impregnated paper, and cardboard. The cellulosic material is derived from wood, reclaimed paper, abaca, jute, or a mixture thereof. A paper bag is particularly preferred.
The paper bag has a plurality of walls, with at least one wall comprising at least two layers of paper. The receptacle and the solid particles together weigh about 11-33 lb (5.0-15.1 kg), preferably about 15-20 lb. (6.8-9.1 kg). The receptacle and the solid particles together provide a covering having a depth of a few inches to several inches over the crust hole, preferably about 2-6 inches (5-15 cm), and about 3 inches (7.5 cm) in a preferred embodiment. Solid particles in the paper bag comprise a flowable mass so that the filled bag is sufficiently flexible to follow the upper contour of the crust.
The electrolytic cell of the invention preferably produces aluminum by electrolyzing alumina dissolved in a molten salt bath electrolyte. Other metals produced by similar electrolytic processes include magnesium, zinc, lithium, and lead.
When the cell produces aluminum by electrolysis of alumina, the solid particles preferably comprise one or more aluminum compounds. Some suitable aluminum compounds includes alumina, aluminum fluoride, cryolite, and mixtures thereof in various proportions. One suitable form of alumina is smelting grade alumina (SGA), typically having an LOI of less than 1 wt. %, 99 m2/g surface area, average pore (SGA), typically having an LOI of less than 1 wt. %, 99 m2/g surface area, average pore volume 0.224 cm3/g, average pore size about 92 microns, and xe2x88x92100+325 mesh size. SGA is commonly used as a feedstock for aluminum smelters. Another suitable form of alumina is electrostatic precipitator dust (ESP dust) obtained from pollution control devices in aluminum smelting plants and in aluminum refineries.